I have tried also kernel 2.6.39-0 from kernel/ppa without result: the problem persist.

I found in /var/log/kern.log 


May  1 01:26:35 anakin kernel: [ 2946.045391] show_signal_msg: 15 callbacks suppressed
May  1 01:26:35 anakin kernel: [ 2946.045395] compiz[5781]: segfault at 4a00014 ip b714f33d sp bfcc8104 error 4 in libpthread-2.13.so[b7147000+15000]
May  1 01:27:49 anakin kernel: [ 3019.650984] compiz[6272]: segfault at 4800014 ip b71c533d sp bfdfa6e4 error 4 in libpthread-2.13.so[b71bd000+15000]
May  1 01:28:34 anakin kernel: [ 3064.848808] kernel tried to execute NX-protected page - exploit attempt? (uid: 0)
May  1 01:28:34 anakin kernel: [ 3064.848811] BUG: unable to handle kernel paging request at c00cb813
May  1 01:28:34 anakin kernel: [ 3064.848813] IP: [<c00cb813>] 0xc00cb813
May  1 01:28:34 anakin kernel: [ 3064.848823] *pdpt = 000000000187d001 *pde = 0000000001958067 *pte = 80000000000cb163 
May  1 01:28:34 anakin kernel: [ 3064.848827] Oops: 0011 [#1] SMP 
May  1 01:28:34 anakin kernel: [ 3064.848829] last sysfs file: /sys/devices/virtual/hwmon/hwmon0/temp1_input






Some time I had this crash (I have just submitted to Bug Report) on shutdown from Indicator Applet, saved in  polkit-gnome-authentication-agent-1-bugreport.txt

System: Linux 2.6.38-8-generic-pae #42-Ubuntu SMP Mon Apr 11 05:17:09 UTC 2011 i686
X Vendor: The X.Org Foundation
X Vendor Release: 11001000
Selinux: No
Accessibility: Disabled
GTK+ Theme: Clearlooks
Icon Theme: ubuntu-mono-light
GTK+ Modules: gnomesegvhandler, canberra-gtk-module

Memory status: size: 112394240 vsize: 112394240 resident: 12398592 share: 9891840 rss: 12398592 rss_rlim: 18446744073709551615
CPU usage: start_time: 1304202894 rtime: 19 utime: 16 stime: 3 cutime:1 cstime: 0 timeout: 0 it_real_value: 0 frequency: 100



----------- .xsession-errors ---------------------
** (gnome-session:2008): DEBUG: GsmDBusClient: obj_path=/org/freedesktop/DBus interface=org.freedesktop.DBus method=NameOwnerChanged
** (gnome-session:2008): DEBUG: GsmDBusClient: obj_path=/org/freedesktop/DBus interface=org.freedesktop.DBus method=NameOwnerChanged
** (gnome-session:2008): DEBUG: GsmDBusClient: obj_path=/org/freedesktop/DBus interface=org.freedesktop.DBus method=NameOwnerChanged
** (gnome-session:2008): DEBUG: GsmAutostartApp: (pid:2080) done (status:0)
** (gnome-session:2008): DEBUG: GsmDBusClient: obj_path=/org/freedesktop/DBus interface=org.freedesktop.DBus method=NameOwnerChanged
** (gnome-session:2008): DEBUG: GsmDBusClient: obj_path=/org/freedesktop/DBus interface=org.freedesktop.DBus method=NameOwnerChanged
** (gnome-session:2008): DEBUG: GsmDBusClient: obj_path=/org/freedesktop/DBus interface=org.freedesktop.DBus method=NameOwnerChanged
** (gnome-session:2008): DEBUG: GsmDBusClient: obj_path=/org/freedesktop/DBus interface=org.freedesktop.DBus method=NameOwnerChanged
** (gnome-session:2008): DEBUG: GsmDBusClient: obj_path=/org/freedesktop/DBus interface=org.freedesktop.DBus method=NameOwnerChanged
Could not attach to process.  If your uid matches the uid of the target
process, check the setting of /proc/sys/kernel/yama/ptrace_scope, or try
again as the root user.  For more details, see /etc/sysctl.d/10-ptrace.conf
ptrace: Operation not permitted.
/home/test/2085: No such file or directory.
No stack.
--------------------------------------------------